I would approach the idea in a slightly different way.
First, define the opposition. I don't think skill challenges work well unless you have
characters on both sides. I mean characters in a story sense; things with an emotional investment in a certain outcome. In that view, the dungeon can be a character, just like Carribean in The Old Man and the Sea.
I think your characters are:
- The PCs, who want to sleep and rest;
- The spirits killed by Sir Keegan, who want vengance upon the living;
- Orcus, as represented by the Shadow Rift, who wants to destroy his enemies;
- and Sir Keegan himself, who wants to be at peace.
Though Sir Keegan is locked up, he might have enough emotional investment in the dungeon environment to play a role. His desire to be at peace ruins the PC's goal because he wants them out of here, they just remind him of his sin, his failure that they have come to correct. However, he could become a source of aid for the PCs, if dealt with in the right way.
The spirits of the slain are going to haunt the PCs. They can be put at rest if their bodies are finally given proper rites.
Orcus and the Shadow Rift - this is going to take the shape of dreams, urgings to kill and slaughter, etc. Basically the same thing that Sir Keegan went through.
So, with that in mind, we come up with a bunch of actions taken by the NPCs that the PCs have to react to.
- Spectral images haunt the party - ghosts float through their campsite, tearing into the PCs, the chill of the grave threatening to spoil their rest and sap the strength from their bones. They cry out: If we cannot rest, neither shall you! (i.e. Throw in their backstory.) Failure: Each PC loses a Healing Surge that he can't get back in this Extended Rest.
- The PCs might be able to track them back to their bodies, at which point they can deal with them. Failure: Stricken by the hopelessness of the situation, their unability to help, they suffer a penalty to future, similar rolls (probably Religion).
- Sir Keegan calls his wife a traitor and a succubus sent by Orcus and he kills her. The PCs watch, horrified by the spectral image playing out before them. Failure: The PCs feel the pain of Keegan's sword blows, hear his wife's cries throughout the night, and lose a Healing Surge they can't recover.
- Finally asleep, they experience unsettling dreams. Dreams that their allies are conspiring against them in the name of Orcus. The only thing to do is to kill them first. Failure: The PC who fails targets another PC who loses a Healing Surge that he can't recover.
- Another encounter with Sir Keegan's spectral ghost.
- A dream where the PCs have fallen in the final room, and Kalarel sacrifices him to the Shadow Rift, loosing the Thing in the Portal onto the world. Failure: Lose a Healing Surge he can't recover, maybe a penalty to a future related roll.
Keep the situation in hand and ever-changing, moving from "minute-to-minute" to much later based on the rolls. Those few ideas should be enough to get the ball rolling, and you play out the response of each character to the actions of the PCs.
You might want to target each PC specifically - only those on watch, for example, or only one has the dream, or only those who leave the campsite (for whatever reason) have to make the roll. If you can tailor the roll to the PC, even better. (A Warlock might be more easily tempted, for example.)